1. Field of the Invention
The present invention relates to a zoom lens and, more particularly, to a zoom lens comprising two lens groups which is particularly suitable for use with a three liquid crystal plate projector.
2. Description of Related Art
Various types of apparatuses have heretofore been used to project a color image on a remote screen. A type of such projection apparatuses have three liquid crystal display elements in the form of a plate for displaying achromatic images. Such a projector is called a "three liquid crystal plate projector (which is hereinafter referred to as a 3CLD projector for simplicity)."
3LCD projectors typically include a projection optical system having a transmitting type of liquid crystal display elements, formed as flat plates, (which are hereinafter referred to as LCD plates for simplicity), for displaying three achromatic component images, corresponding to primary monochromatic component images; namely, red, green and blue component images, of an intended color image. The achromatic component images displayed on the liquid crystal display plates are translated into three monochromatic component images, namely, red, green and blue component images, by the use of three dichroic mirrors. The red, green and blue component images are then composed and projected by a projection lens as a color image on a remote screen.
In a projection optical system of the 3LCD projector, since there must be provided a number of optical mirror elements, for instance, at least three dichroic mirrors and three reflection mirrors, as well as three LCD plates, the projection lens must have a long back focal length between the projection lens and each of the LCD plates.
Zoom lenses comprising, for instance, four or five lens groups are preferably available as projection lenses having a relatively long back focal length and desirable optical characteristics. Such a zoom lens, however, unavoidably has a long overall length and a large diameter of a foremost lens. This leads to a 3LCD projector of a large size.
There has been proposed, as an improved projection lens, a zoom lens having a reduced number of, for instance two, lens groups and a long back focal length. Such is described in Japanese Unexamined Patent Publication No. 3-120,507. This zoom lens satisfies the following conditions: EQU bf.sub.w /f.sub.w &gt;1.6 EQU 0.8&lt;-e.sub.w /f.sub.I &lt;2 EQU 1&lt;-f.sub.I /f.sub.II &lt;1.5
where
bf.sub.w is the rear vertex distance measured from the rear surface of the rearmost lens component of the second lens group and a rear conjugate point for the wide-angle end;
f.sub.w is the overall focal length of the zoom lens for the wide-angle end;
f.sub.I is the focal length of the first lens group;
f.sub.II is the focal length of the second lens group; and
e.sub.w is the distance between a principal point of the first lens group and a principal point of the second lens group.
In order to allow viewers or spectators to see an image projected on a remote screen by a 3LCD projector at any desired location on a floor, it is generally preferred to suspend the 3LCD projector from the ceiling rather than to place it on the floor or, otherwise, on a table. For this reason, the 3LCD projector desirably has a horizontal arrangement of optical elements, in which dichroic mirrors and reflection mirrors are disposed in two parallel straight lines separated horizontally, so as to have a flat and thin shape.
However, because screens are generally oblong in geometry and, when the projector has a horizontal optical arrangement, each of the optical elements must be oblong in geometry, an optical path has a long distance between the rearmost surface of the optical system of the projection lens and each CLD plate. Consequently, the projection lens must have a long back focal length.
If the zoom lens described in the publication mentioned above is used with a 3LCD projector having a horizontal arrangement of projection optical elements, the back focal length of the zoom lens is not always sufficiently long.